Most of the kernel code is written inC as supported by theGNU Compiler Collection (GCC) which has extensions beyond standard C.[14]: 18 [15] The code also containsassembly code for architecture-specific logic such as optimizing memory use and task execution.[14]: 379–380 The kernel has amodular design such that modules can be integrated assoftware components – including dynamically loaded. The kernel ismonolithic in an architectural sense since the entire OS kernel runs inkernel space.
I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486)AT clones. This has been brewing since April, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things). I've currently portedbash(1.08) andgcc(1.40), and things seem to work. This implies that I'll get something practical within a few months [...] Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT protable [sic] (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.
On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network (FUNET). It was not even executable since its code still needed Minix to compile and test it.[18]
On 5 October 1991, Torvalds announced the first "official" version of Linux, version 0.02.[19][18]
[As] I mentioned a month ago, I'm working on a free version of a Minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02...but I've successfully run bash, gcc, gnu-make, gnu-sed, compress, etc. under it.
Linux grew rapidly as many developers, including theMINIX community, contributed to the project.[citation needed] At the time, theGNU Project had completed many components for its free UNIX replacement,GNU, but its kernel, theGNU Hurd, was incomplete. The project adopted the Linux kernel for its OS.[20]
Torvalds labeled the kernel with major version 0 to indicate that it was not yet intended for general use.[21] Version 0.11, released in December 1991, was the first version to beself-hosted; compiled on a computer running the Linux kernel.
When Torvalds released version 0.12 in January 1992, he adopted theGNU General Public License version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution. GPL took effect as of 1 February 1992.[22] In contrast toUnix, allsource files of Linux are freely available, includingdevice drivers.[23]
The initial success of Linux was driven by programmers and testers across the world. With the support of thePOSIX APIs, through the libC that, whether needed, acts as an entry point to the kernel address space, Linux could run software and applications that had been developed for Unix.[24]
The Linux kernel supports various hardware architectures, providing a common platform for software, includingproprietary software.
On 19 January 1992, the first post to the new newsgroupalt.os.linux was submitted.[25] On 31 March 1992, the newsgroup was renamedcomp.os.linux.[26]
Version 0.96 released in May 1992 was the first capable of running theX Window System.[30][31] In March 1994, Linux 1.0.0 was released with 176,250 lines of code.[32] As indicated by the version number, it was the first version considered suitable for aproduction environment.[21] In June 1996, after release 1.3, Torvalds decided that Linux had evolved enough to warrant a new major number, and so labeled the next release as version 2.0.0.[33][34] Significant features of 2.0 includedsymmetric multiprocessing (SMP), support for more processors types and support for selecting specific hardware targets and for enabling architecture-specific features and optimizations.[24] Themake *config family of commands ofkbuild enable and configure options for building ad hoc kernel executables (vmlinux) and loadable modules.[35][36]
Version 2.2, released on 20 January 1999,[37] improved locking granularity and SMP management, addedm68k,PowerPC,Sparc64,Alpha, and other 64-bit platforms support.[38] Furthermore, it added newfile systems includingMicrosoft'sNTFS read-only capability.[38] In 1999, IBM published its patches to the Linux 2.2.13 code for the support of theS/390 architecture.[39]
Version 2.4.0, released on 4 January 2001,[40] contained support forISAPlug and Play,USB, andPC Cards. Linux 2.4 added support for thePentium 4 andItanium (the latter introduced theia64 ISA that was jointly developed by Intel and Hewlett-Packard to supersede the olderPA-RISC), and for the newer64-bit MIPS processor.[41] Development for 2.4.x changed a bit in that more features were made available throughout the series, including support forBluetooth,Logical Volume Manager (LVM) version 1,RAID support,InterMezzo andext3 file systems.
Version 2.6.0 was released on 17 December 2003.[42] The development for 2.6.x changed further towards including new features throughout the series. Among the changes that have been made in the 2.6 series are: integration ofμClinux into the mainline kernel sources,PAE support, support for several new lines ofCPUs, integration of Advanced Linux Sound Architecture (ALSA) into the mainline kernel sources, support for up to 232 users (up from 216), support for up to 229 process IDs (64-bit only, 32-bit architectures still limited to 215),[43] substantially increased the number of device types and the number of devices of each type, improved64-bit support, support forfile systems which support file sizes of up to 16terabytes, in-kernelpreemption, support for theNative POSIX Thread Library (NPTL),User-mode Linux integration into the mainline kernel sources,SELinux integration into the mainline kernel sources,InfiniBand support, and considerably more.
Though development had not used aversion control system thus far, in 2002, Linux developers adoptedBitKeeper, which was made freely available to them even though it was notfree software. In 2005, because of efforts toreverse-engineer it, the company which owned the software revoked its support of the Linux community. In response, Torvalds and others wroteGit. The new system was written within weeks, and in two months the first official kernel made using it was released.[46]
In 2005 thestable team was formed as a response to the lack of a kernel tree where people could work onbug fixes, and it would keep updatingstable versions.[47] In February 2008 thelinux-next tree was created to serve as a place where patches aimed to be merged during the next development cycle gathered.[48][49] Several subsystem maintainers also adopted the suffix-next for trees containing code which they mean to submit for inclusion in the next release cycle. As of January 2014[update], the in-development version of Linux is held in an unstable branch namedlinux-next.[50]
The 20th anniversary of Linux was celebrated by Torvalds in July 2011 with the release of version 3.0.0.[33] As 2.6 had been the version number for 8 years, a newuname26 personality that reports 3.x as 2.6.40+x had to be added to the kernel so that old programs would work.[51]
Version 3.0 was released on 22 July 2011.[52] On 30 May 2011, Torvalds announced that the big change was "NOTHING. Absolutely nothing." and asked, "...let's make sure we really make the next release not just an all new shiny number, but a good kernel too."[53] After the expected 6–7 weeks of the development process, it would be released near the 20th anniversary of Linux.
On 11 December 2012, Torvalds decided to reduce kernel complexity by removing support fori386 processors—specifically by not having toemulate[54] theatomicCMPXCHG instruction introduced with thei486 to allow reliablemutexes—making the 3.7 kernel series the last one still supporting the original processor.[55][56] The same series unified support for theARM processor.[57]
The numbering change from 2.6.39 to 3.0, and from 3.19 to 4.0, involved no meaningful technical differentiation; the major version number was increased simply to avoid large minor numbers.[52][58] Stable 3.x.y kernels were released until 3.19 in February 2015. Version 3.11, released on 2 September 2013,[59] added many new features such as newO_TMPFILE flag foropen(2) to reduce temporary file vulnerabilities, experimental AMDRadeon dynamic power management, low-latency network polling, andzswap (compressed swap cache).[60]
In April 2015, Torvalds released kernel version 4.0.[33] By February 2015, Linux had received contributions from nearly 12,000 programmers from more than 1,200 companies, including some of the world's largest software and hardware vendors.[61] Version 4.1 of Linux, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers.[62]
Linus Torvalds announced that kernel version 4.22 would instead be numbered 5.0 in March 2019, stating that "'5.0' doesn't mean anything more than that the 4.x numbers started getting big enough that I ran out of fingers and toes."[63] It featured many major additions such as support for the AMD RadeonFreeSync andNVIDIA Xavier display, fixes forF2FS,EXT4 andXFS, restored support for swap files on theBtrfsfile system and continued work on theIntelIcelake Gen11 graphics and on theNXPi.MX8 SoCs.[64][65] This release was noticeably larger than the rest, Torvalds mentioning that "The overall changes for all of the 5.0 release are much bigger."[63]
A total of 1,991 developers, of whom 334 were first-time collaborators, added more than 553,000 lines of code to version 5.8, breaking the record previously held by version 4.9.[66]
According to the Stack Overflow's annual Developer Survey of 2019, more than 53% of all respondents have developed software forLinux and about 27% forAndroid,[67] although only about 25% develop with Linux-based operating systems.[68]
Android, which runs on a modified Linux kernel, accounts for the majority of mobile device operating systems,[72][73][74] and is increasingly being used inembedded devices, making it a significant driver of Linux adoption.[24]
The cost to redevelop version 2.6.0 of the Linux kernel in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using theCOCOMO person-month estimation model.[75] In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882M ($1.14bn, £744M).[76]
This topic was revisited in October 2008 by Amanda McPherson, Brian Proffitt, and Ron Hale-Evans. Using David A. Wheeler's methodology, they estimated redevelopment of the 2.6.25 kernel now costs $1.3bn (part of a total $10.8bn to redevelop Fedora 9).[77] Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union.[78]
As of 7 March 2011[update], using then-currentLOC (lines of code) of a 2.6.x Linux kernel and wage numbers with David A. Wheeler's calculations it would cost approximately $3bn (about €2.2bn) to redevelop the Linux kernel as it keeps getting bigger. An updated calculation as of 26 September 2018[update], using then-current 20,088,609 LOC (lines of code) for the 4.14.14 Linux kernel and the current US national average programmer salary of $75,506 show that it would cost approximately $14,725,449,000 (£11,191,341,000) to rewrite the existing code.[79]
Most who use Linux do so via aLinux distribution. Some distributions ship the vanilla or stable kernel. However, several vendors (such asRed Hat andDebian) maintain a customized source tree. These are usually updated at a slower pace than the vanilla branch, and they usually include all fixes from the relevant stable branch, but at the same time they can also add support for drivers or features which had not been released in the vanilla version the distribution vendor started basing its branch from.
Graph of the sizes of Linux Kernel versions in millions of lines of code[80]. Viewsource data.
The community of Linux kernel developers comprises about 5000–6000 members. According to the "2017 State of Linux Kernel Development", a study issued by the Linux Foundation, covering the commits for the releases 4.8 to 4.13, about 1500 developers were contributing from about 200–250 companies on average. The top 30 developers contributed a little more than 16% of the code. For companies, the top contributors are Intel (13.1%) and Red Hat (7.2%), Linaro (5.6%), IBM (4.1%), the second and fifth places are held by the 'none' (8.2%) and 'unknown' (4.1%) categories.[81]
"Instead of a roadmap, there are technical guidelines. Instead of a central resource allocation, there are persons and companies who all have a stake in the further development of the Linux kernel, quite independently from one another:People like Linus Torvalds and I don’t plan the kernel evolution. We don’t sit there and think up the roadmap for the next two years, then assign resources to the various new features. That's because we don’t have any resources. The resources are all owned by the various corporations who use and contribute to Linux, as well as by the various independent contributors out there. It's those people who own the resources who decide..."
In July 2007,Con Kolivas announced that he would cease developing for the Linux kernel.[82][83]
In July 2009,Alan Cox quit his role as theTTY layer maintainer after disagreement with Torvalds.[84]
In December 2010, there was a discussion between Linux SCSI maintainer James Bottomley and SCST maintainer Vladislav Bolkhovitin about which SCSI target stack should be included in the Linux kernel.[85] This made some Linux users upset.[86]
In June 2012, Torvalds made it very clear that hedid not agree withNVIDIA releasing its drivers as closed.[87]
In April 2014, Torvalds bannedKay Sievers from submitting patches to the Linux kernel for failing to deal withbugs that causedsystemd to negatively interact with the kernel.[88]
In October 2014,Lennart Poettering accused Torvalds of tolerating the rough discussion style on Linux kernel related mailing lists and of being a bad role model.[89]
In March 2015, Christoph Hellwig filed a lawsuit againstVMware for infringement of the copyright on the Linux kernel.[90] Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease.[91]
In April 2021, a team from theUniversity of Minnesota was found to be submitting "bad faith" patches to the kernel as part of its research. This resulted in the immediate reversion of all patches ever submitted by a member of the university. In addition, a warning was issued by a senior maintainer that any future patch from the university would be rejected on sight.[92][93]
Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers.[94] For a long time there was no code of conduct for kernel developers due to opposition by Torvalds.[95] However, a Linux KernelCode of Conflict was introduced on 8 March 2015.[96] It was replaced on 16 September 2018 by a newCode of Conduct based on theContributor Covenant. This coincided with a public apology by Torvalds and a brief break from kernel development.[97][98] On 30 November 2018, complying with theCode of Conduct, Jarkko Sakkinen of Intel sent out patches replacing instances of "fuck" appearing in source code comments with suitable versions focused on the word 'hug'.[99]
Developers who feel treated unfairly can report this to theLinux Foundation Technical Advisory Board.[100] In July 2013, the maintainer of the USB 3.0 driverSage Sharp asked Torvalds to address the abusive commentary in the kernel development community. In 2014, Sharp backed out of Linux kernel development, saying that "The focus on technical excellence, in combination with overloaded maintainers, and people with different cultural and social norms, means that Linux kernel maintainers are often blunt, rude, or brutal to get their job done".[101] At the linux.conf.au (LCA) conference in 2018, developers expressed the view that the culture of the community has gotten much better in the past few years. Daniel Vetter, the maintainer of the Intel drm/i915 graphics kernel driver, commented that the "rather violent language and discussion" in the kernel community has decreased or disappeared.[102]
Laurent Pinchart asked developers for feedback on their experiences with the kernel community at the 2017 Embedded Linux Conference Europe. The issues brought up were discussed a few days later at the Maintainers Summit. Concerns over the lack of consistency in how maintainers responded to patches submitted by developers were echoed byShuah Khan, the maintainer of the kernel self-test framework. Torvalds contended that there would never be consistency in the handling of patches because different kernel subsystems have, over time, adopted different development processes. Therefore, it was agreed upon that each kernel subsystem maintainer would document the rules for patch acceptance.[103]
As of 2021[update], the 5.11 release of the Linux kernel had around 30.34 million lines of code. Roughly 14% of the code is part of the "core," including architecture-specific code, kernel code, and memory management code, while 60% is drivers.
Contributions are submitted as patches, in the form of text messages on theLinux kernel mailing list (LKML) (and often also on other mailing lists dedicated to particular subsystems). The patches must conform to a set of rules and to a formal language that, among other things, describes which lines of code are to be deleted and what others are to be added to the specified files. These patches can be automatically processed so that system administrators can apply them in order to make just some changes to the code or to incrementally upgrade to the next version.[108] Linux is distributed also inGNU zip (gzip) andbzip2 formats.
A developer who wants to change the Linux kernel writes and tests a code change. Depending on how significant the change is and how many subsystems it modifies, the change will either be submitted as a single patch or in multiple patches ofsource code. In case of a single subsystem that is maintained by a single maintainer, these patches are sent as e-mails to the maintainer of the subsystem with the appropriate mailing list in Cc. The maintainer and the readers of the mailing list will review the patches and provide feedback. Once the review process has finished the subsystem maintainer accepts the patches in the relevantGit kernel tree. If the changes to the Linux kernel are bug fixes that are considered important enough, a pull request for the patches will be sent to Torvalds within a few days. Otherwise, a pull request will be sent to Torvalds during the next merge window. The merge window usually lasts two weeks and starts immediately after the release of the previous kernel version.[109] The Git kernel source tree names all developers who have contributed to the Linux kernel in theCredits directory and all subsystem maintainers are listed inMaintainers.[110]
As with many large open-source software projects, developers are required to adhere to theContributor Covenant, acode of conduct intended to address harassment of minority contributors.[111][112] Additionally, to prevent offense the use ofinclusive terminology within the source code is mandated.[113]
In September 2021, the GCC version requirement for compiling and building the Linux kernel increased from GCC 4.9 to 5.1, allowing the potential for the kernel to be moved from using C code based on theC89 standard to using code written with theC11 standard,[114] with the migration to the standard taking place in March 2022, with the release of Linux 5.18.[115]
Initial support for theRust programming language was added in Linux 6.1[116] which was released in December 2022,[117] with later kernel versions, such as Linux 6.2 and Linux 6.3, further improving the support.[118][119]
As for most software, the kernel is versioned as a series of dot-separated numbers.
For early versions, the version consisted of three or four dot-separated numbers calledmajor release,minor release andrevision.[14]: 9 At that time, odd-numbered minor releases were for development and testing, while even numbered minor releases for production. The optional fourth digit indicated a patch level.[21] Development releases were indicated with a release candidate suffix (-rc).
The current versioning conventions are different. The odd/even number implying dev/prod has been dropped, and a major version is indicated by the first two numbers together. While the time-frame is open for the development of the next major, the -rcN suffix is used to identify the n'th release candidate for the next version.[122] For example, the release of the version 4.16 was preceded by seven 4.16-rcN (from -rc1 to -rc7). Once a stable version is released, its maintenance is passed to thestable team. Updates to a stable release are identified by a three-number scheme (e.g., 4.16.1, 4.16.2, ...).[122]
GNU cc was for a long time the only compiler capable of correctly building Linux. In 2004,Intel claimed to have modified the kernel so thatits C compiler was also capable of compiling it.[123] There was another such reported success in 2009, with a modified 2.6.22 version.[124][125] Support for the Intel compiler has been dropped in 2023.[126]
Since 2010, effort has been underway to build Linux withClang, an alternative compiler for the C language;[127] as of 12 April 2014, the official kernel could almost be compiled by Clang.[128][129] The project dedicated to this effort is namedLLVMLinux after theLLVM compiler infrastructure upon which Clang is built.[130] LLVMLinux does not aim to fork either Linux or the LLVM, therefore it is a meta-project composed of patches that are eventually submitted to the upstream projects. By enabling Linux to be compiled by Clang, developers may benefit from shorter compilation times.[131]
In 2017, developers completed upstreaming patches to support building the Linux kernel withClang in the 4.15 release, havingbackported support forX86-64 andAArch64 to the 4.4, 4.9, and 4.14 branches of the stable kernel tree. Google'sPixel 2 shipped with the firstClang builtLinux kernel,[132] though patches forPixel (1st generation) did exist.[133] 2018 sawChromeOS move to building kernels withClang by default,[134] whileAndroid madeClang[135] andLLVM's linker LLD[136] required for kernel builds in 2019.Google moved its production kernel used throughout its datacenters to being built withClang in 2020.[137] TheClangBuiltLinux group coordinates fixes to bothLinux andLLVM to ensure compatibility, both composed of members fromLLVMLinux and having upstreamed patches fromLLVMLinux.
As with any software, problems with the Linux kernel can be difficult totroubleshoot. Common challenges relate to userspace vs. kernel space access, misuse of synchronization primitives, and incorrect hardware management.[14]: 364
Anoops is a non-fatal error in the kernel. After such an error, operations continue with suspect reliability.[138]
A panic (generated bypanic()) is a fatal error. After such an error, the kernel prints a message and halts the computer.[14]: 371
The kernel provides fordebugging by printing viaprintk() which stores messages in a circular buffer (overwriting older entries with newer). Thesyslog(2) system call provides for reading and clearing the message buffer and for setting the maximumlog level of the messages to be sent to the console.[139] Kernel messages are also exported to userland through the/dev/kmsg interface.[140]
Theftrace mechanism allow for debugging by tracing. It is used for monitoring and debugging Linux at runtime and it can analyze user space latencies due to kernel misbehavior.[141][142][143][144] Furthermore,ftrace allows users to trace Linux at boot-time.[145]
kprobes andkretprobes can break into kernel execution (like debuggers in userspace) and collect information non-disruptively.[146]kprobes can be inserted into code at (almost) any address, while kretprobes work at function return.uprobes have similar purposes but they also have some differences in usage and implementation.[147]
WithKGDB Linux can be debugged in much the same way as userspace programs. KGDB requires an additional machine that runsGDB and that is connected to the target to be debugged using aserial cable orEthernet.[148]
The Linux kernel project integrates new code on a rolling basis. Standard operating procedure is that software checked into the project must work andcompile without error.
Each kernel subsystem is assigned a maintainer who is responsible for reviewing patches against the kernel code standards and keeping a queue of patches that can be submitted to Torvalds within a merge window that is usually several weeks.
Patches are merged by Torvalds into the source code of the prior stable Linux kernel release, creating the release candidate (-rc) for the next stable release. Once the merge window is closed, only fixes to the new code in the development release are accepted. The -rc development release of the kernel goes throughregression testing and once it is considered stable by Torvalds and the subsystem maintainers, a new version is released and the development process starts over again.[149]
The Git tree that contains the Linux kernel source code is referred to asmainline Linux. Every stable kernel release originates from the mainline tree,[150] and is frequently published onkernel.org. Mainline Linux has only solid support for a small subset of the many devices that run Linux. Non-mainline support is provided by independent projects, such asYocto orLinaro, but in many cases the kernel from the device vendor is needed.[151] Using a vendor kernel likely requires aboard support package.
Maintaining a kernel tree outside of mainline Linux has proven to be difficult.[152]
Mainlining refers to the effort of adding support for a device to the mainline kernel,[153] while there was formerly only support in a fork or no support at all. This usually includes adding drivers ordevice tree files. When this is finished, the feature or security fix is consideredmainlined.[154]
The maintainer of the stable branch,Greg Kroah-Hartman, has applied the termLinux-like todownstream kernel forks by vendors that add millions of lines of code to the mainline kernel.[155] In 2019,Google stated that it wanted to use the mainline Linux kernel inAndroid so the number of kernel forks would be reduced.[156] The term Linux-like has also been applied to theEmbeddable Linux Kernel Subset, which does not include the full mainline Linux kernel but a small modified subset of the code.[157]
There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from theseforks that includeLinux-libre,Compute Node Linux,INK,L4Linux,RTLinux, andUser-Mode Linux (UML) have been merged into the mainline.[158] Some operating systems developed for mobile phones initially used heavily modified versions of Linux, including GoogleAndroid,Firefox OS, HPwebOS, NokiaMaemo and JollaSailfish OS. In 2010, the Linux community criticised Google for effectively starting its own kernel tree:[159][160]
This means that any drivers written for Android hardware platforms, can not get merged into the main kernel tree because they have dependencies on code that only lives in Google's kernel tree, causing it to fail to build in the kernel.org tree. Because of this, Google has now prevented a large chunk of hardware drivers and platform code from ever getting merged into the main kernel tree. Effectively creating a kernel branch that a number of different vendors are now relying on.[161]
Today Android uses a customized Linux[162] where major changes are implemented in device drivers, but some changes to the core kernel code is required. Android developers also submit patches to the official Linux that finally can boot the Android operating system. For example, aNexus 7 can boot and run the mainline Linux.[162]
At a 2001 presentation at theComputer History Museum, Torvalds had this to say in response to a question about distributions of Linux using precisely the same kernel sources or not:
They're not... well they are, and they're not. There is no single kernel. Every single distribution has their own changes. That's been going on since pretty much day one. I don't know if you may rememberYggdrasil was known for having quite extreme changes to the kernel and even today all of the major vendors have their own tweaks because they have some portion of the market they're interested in and quite frankly that's how it should be. Because if everybody expects one person, me, to be able to track everything that's not the point of GPL. That's not the point of having an open system. So actually the fact that a distribution decides that something is so important to them that they will add patches for even when it's not in the standard kernel, that's a really good sign for me. So that's for example how something likeReiserFS got added. And the reason why ReiserFS is the first journaling filesystem that was integrated in the standard kernel was not because I loveHans Reiser. It was becauseSUSE actually started shipping with ReiserFS as their standard kernel, which told me "ok." This is actually in production use. Normal People are doing this. They must know something I don't know. So in a very real sense what a lot of distribution houses do, they are part of this "let's make our own branch" and "let's make our changes to this." And because of the GPL, I can take the best portions of them.[163]
The latest version and older versions are maintained separately. Most of the latest kernel releases were supervised by Torvalds.[164]
The Linux kernel developer community maintains a stable kernel by applying fixes forsoftware bugs that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org always lists two stable kernels. The next stable Linux kernel is released about 8 to 12 weeks later.
Some projects have attempted to reduce the size of the Linux kernel. One of them isTinyLinux. In 2014, Josh Triplett started the -tiny source tree for a reduced size version.[166][167][168][169]
Map of the Linux kernelSankey diagram of Linux Kernel Source Lines of Code
Even though seemingly contradictory, the Linux kernel is both monolithic and modular. The kernel is classified as amonolithic kernel architecturally since the entire OS runs in kernel space. The design is modular since it can be assembled frommodules that in some cases are loaded and unloaded at runtime.[14]: 338 [170] It supports features once only available in closed source kernels of non-free operating systems.
The rest of the article makes use of the UNIX and Unix-like operating systems convention of themanual pages. The number that follows the name of a command, interface, or other feature specifies the section (i.e. the type of the OS' component or feature) it belongs to. For exampleexecve(2) refers to a system call, andexec(3) refers to a userspace library wrapper.
The following is an overview of architectural design and of noteworthy features.
Selection and configuration of hundreds of kernel features and drivers (using one of themake *config family of commands before building),[171][36][35] modification of kernel parameters beforeboot (usually by inserting instructions into the lines of theGRUB2 menu), and fine tuning of kernel behavior at run-time (using thesysctl(8) interface to/proc/sys/).[172][173][174]
Configurable I/O schedulers,ioctl(2)[181] syscall that manipulates the underlying device parameters of special files (it is a non standard system call, since arguments, returns, and semantics depends on the device driver in question), support for POSIX asynchronous I/O[182] (however, because they scale poorly with multithreaded applications, a family of Linux specific I/O system calls (io_*(2)[183]) had to be created for the management of asynchronous I/O contexts suitable for concurrent processing).
I/O Virtualization withVFIO andSR-IOV. Virtual Function I/O (VFIO) exposes direct device access to user space in a secure memory (IOMMU) protected environment. With VFIO, a VM Guest can directly access hardware devices on the VM Host Server. This technique improves performance, if compared both to Full virtualization and Paravirtualization. However, with VFIO, devices cannot be shared with multiple VM guests. Single Root I/O Virtualization (SR-IOV) combines the performance gains of VFIO and the ability to share a device with several VM Guests (but it requires special hardware that must be capable to appear to two or more VM guests as different devices).[191]
Unlike standard monolithic kernels, device drivers are easily configured asmodules, and loaded or unloaded while the system is running and can also be pre-empted under certain conditions in order to handlehardware interrupts correctly and to better supportsymmetric multiprocessing.[177] By choice, Linux has no stable device driverapplication binary interface.[196]
The hardware is represented in the file hierarchy. User applications interact with device drivers via entries in the/dev or/sys directories.[199] Process information is mapped into the/proc directory.[199]
Various layers within Linux, also showing separation between theuserland andkernel space
Four interfaces are distinguished: two internal to the kernel, and two between the kernel and userspace.
Linux started as a clone of UNIX, and aims towardPOSIX andSingle UNIX Specification compliance.[201] The kernel provides system calls and other interfaces that are Linux-specific. In order to be included in the official kernel, the code must comply with a set of licensing rules.[8][13]
Loadable kernel modules (LKMs), by design, cannot rely on a stable ABI.[196] Therefore, they must always be recompiled whenever a new kernel executable is installed in a system, otherwise they will not be loaded. In-tree drivers that are configured to become an integral part of the kernel executable (vmlinux) are statically linked by the build process.
There is no guarantee of stability of source-level in-kernel API[196] and, because of this,device driver code, as well as the code of any other kernel subsystem, must be kept updated with kernel evolution. Any developer who makes an API change is required to fix any code that breaks as the result of their change.[204]
The set of theLinux kernel API that regards the interfaces exposed to user applications is fundamentally composed of UNIX and Linux-specificsystem calls.[205] A system call is an entry point into the Linux kernel.[206] For example, among the Linux-specific ones there is the family of theclone(2) system calls.[207] Most extensions must be enabled by defining the_GNU_SOURCEmacro in aheader file or when the user-land code is being compiled.[208]
System calls can only be invoked via assembly instructions that enable the transition from unprivileged user space to privileged kernel space inring 0. For this reason, theC standard library (libC) acts as a wrapper to most Linux system calls, by exposing C functions that, if needed,[209] transparently enter the kernel which will execute on behalf of the calling process.[205] For system calls not exposed by libC, such as thefast userspace mutex,[210] the library provides a function calledsyscall(2) which can be used to explicitly invoke them.[211]
Pseudo filesystems (e.g., thesysfs andprocfs filesystems) andspecial files (e.g.,/dev/random,/dev/sda,/dev/tty, and many others) constitute another layer of interface to kernel data structures representing hardware or logical (software) devices.[212][213]
Because of the differences existing between the hundreds of various implementations of the Linux OS, executable objects, even though they are compiled, assembled, and linked for running on a specific hardware architecture (that is, they use theISA of the target hardware), often cannot run on different Linux distributions. This issue is mainly due to distribution-specific configurations and a set of patches applied to the code of the Linux kernel, differences in system libraries, services (daemons), filesystem hierarchies, and environment variables.
The main standard concerning application and binary compatibility of Linux distributions is theLinux Standard Base (LSB).[214][215] However, the LSB goes beyond what concerns the Linux kernel, because it also defines the desktop specifications, the X libraries and Qt that have little to do with it.[216] The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open,File System Hierarchy (FHS), and others).[217]
The standard ABI for how x86_64 user programs invoke system calls is to load the syscall number into therax register, and the other parameters intordi,rsi,rdx,r10,r8, andr9, and finally to put thesyscall assembly instruction in the code.[225][226][227]
At XDC2014, Alex Deucher from AMD announced the unified kernel-mode driver.[228] The proprietary Linux graphic driver,libGL-fglrx-glx, will share the sameDRM infrastructure withMesa 3D. As there is no stable in-kernelABI, AMD had to constantly adapt the formerbinary blob used by Catalyst.
There are several internal kernel APIs between kernel subsystems. Some are available only within the kernel subsystems, while a somewhat limited set of in-kernel symbols (i.e., variables, data structures, and functions) is exposed to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with theEXPORT_SYMBOL() andEXPORT_SYMBOL_GPL() macros[229][230] (the latter reserved to modules released under a GPL-compatible license).[231]
Linux provides in-kernel APIs that manipulate data structures (e.g.,linked lists,radix trees,[232]red-black trees,[233]queues) or perform common routines (e.g., copy data from and to user space, allocate memory, print lines to the system log, and so on) that have remained stable at least since Linux version 2.6.[234][235][236]
In-kernel APIs include libraries of low-level common services used by device drivers:
SCSI Interfaces andlibATA – respectively, a peer-to-peer packet based communication protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, ATAPI device,[237] and an in-kernel library to support [S]ATA host controllers and devices.[238]
Direct Rendering Manager (DRM) andKernel Mode Setting (KMS) – for interfacing with GPUs and supporting the needs of modern 3D-accelerated video hardware,[239] and for setting screen resolution, color depth and refresh rate[240]
DMA buffers (DMA-BUF) – for sharing buffers for hardware direct memory access across multiple device drivers and subsystems[241][242][243]
The Linux developers chose not to maintain a stable in-kernel ABI. Modules compiled for a specific version of the kernel cannot be loaded into another version without being recompiled.[196]
Linux, as other kernels, has the ability to manage processes including creating, suspending, resuming and terminating. Unlike other operating systems, the Linux kernel implements processes as a group of threads called tasks. If two tasks share the same TGID, then they are called in the kernel terminology a task group. Each task is represented by atask_structdata structure. When a process is created it is assigned a globally unique identifier calledPID and cannot be shared[246][247]
A new process can be created by calling clone[248] family of system calls or forksystem call. Processes can be suspended and resumed by the kernel by sending signals like SIGSTOP and SIGCONT. A process can terminate itself by calling exit system call, or terminated by another process by sending signals like SIGKILL, SIGABRT or SIGINT.
If the executable is dynamically linked to shared libraries, adynamic linker is used to find and load the needed objects, prepare the program to run and then run it.[249]
TheNative POSIX Thread Library (NPTL)[250] provides the POSIX standard thread interface (pthreads) to userspace. The kernel isn't aware of processes nor threads but it is aware oftasks, thus threads are implemented in userspace. Threads in Linux are implemented astasks sharing resources, while if they aren't sharing called to be independent processes.
The kernel provides thefutex(7) (fast user-space mutex) mechanisms for user-space locking and synchronization.[251] The majority of the operations are performed in userspace but it may be necessary to communicate with the kernel using thefutex(2) system call.[210]
As opposed to userspace threads described above,kernel threads run in kernel space.[252] They are threads created by the kernel itself for specialized tasks; they are privileged like the kernel and aren't bound to any process or application.
The Linuxprocess scheduler is modular, in the sense that it enables different scheduling classes and policies.[253][254] Scheduler classes are plugable scheduler algorithms that can be registered with the base scheduler code. Each class schedules different types of processes. The core code of the scheduler iterates over each class in order of priority and chooses the highest priority scheduler that has a schedulable entity of type struct sched_entity ready to run.[14]: 46–47 Entities may be threads, group of threads, and even all the processes of a specific user.
For normal tasks, by default, the kernel uses theCompletely Fair Scheduler (CFS) class[needs update], introduced in version 2.6.23.[179] The scheduler is defined as a macro in a C header asSCHED_NORMAL. In other POSIX kernels, a similar policy known asSCHED_OTHER allocates CPU timeslices (i.e, it assigns absolute slices of the processor time depending on either predetermined or dynamically computed priority of each process). The Linux CFS does away with absolute timeslices and assigns a fair proportion of CPU time, as a function of parameters like the total number of runnable processes and the time they have already run; this function also takes into account a kind of weight that depends on their relative priorities (nice values).[14]: 46–50
With user preemption, the kernel scheduler can replace the current process with the execution of acontext switch to a different one that therefore acquires the computing resources for running (CPU, memory, and more). It makes it according to the CFS algorithm (in particular, it uses a variable calledvruntime for sorting entities and then chooses the one that has the smaller vruntime, - i.e., the schedulable entity that has had the least share of CPU time), to the active scheduler policy and to the relative priorities.[256] With kernel preemption, the kernel can preempt itself when an interrupt handler returns, when kernel tasks block, and whenever a subsystem explicitly calls the schedule() function.
The kernel also contains two POSIX-compliant[257] real-time scheduling classes namedSCHED_FIFO (realtimefirst-in-first-out) andSCHED_RR (realtimeround-robin), both of which take precedence over the default class.[253] An additional scheduling policy known asSCHED DEADLINE, implementing theearliest deadline first algorithm (EDF), was added in kernel version 3.14, released on 30 March 2014.[258][259]SCHED_DEADLINE takes precedence over all the other scheduling classes.
Real-timePREEMPT_RT patches, included into the mainline Linux since version 2.6, provide adeterministic scheduler, the removal of preemption and interrupt disabling (where possible), PI Mutexes (i.e., locking primitives that avoid priority inversion),[260][261] support forHigh Precision Event Timers (HPET), preemptiveread-copy-update (RCU), (forced) IRQ threads, and other minor features.[262][263][264]
The kernel has different causes of concurrency (e.g., interrupts, bottom halves, preemption of kernel and users tasks, symmetrical multiprocessing).[14]: 167
PREEMPT_RT code included in mainline Linux provideRT-mutexes, a special kind of Mutex which do not disable preemption and have support for priority inheritance.[277][278] Almost all locks are changed into sleeping locks when using configuration for realtime operation.[279][264][278]Priority inheritance avoids priority inversion by granting a low-priority task which holds a contended lock the priority of a higher-priority waiter until that lock is released.[280][281]
Linux includes a kernel lock validator calledLockdep.[282][283]
Although the management ofinterrupts could be seen as a single job, it is divided into two. This split in two is due to the different time constraints and to the synchronization needs of the tasks whose the management is composed of. The first part is made up of an asynchronousinterrupt service routine (ISR) that in Linux is known as thetop half, while the second part is carried out by one of three types of the so-calledbottom halves (softirq,tasklets, andwork queues).[14]: 133–137
Linux interrupt service routines can be nested. A new IRQ can trap into a high priority ISR that preempts any other lower priority ISR.
ZONE_HIGHMEM: part of physical memory that is only accessible to the kernel using temporary mapping.
Those zones are the most common, but others exist.[284]
Linux implementsvirtual memory with 4 or 5-levelpage tables.[285] The kernel is notpageable (meaning it is always resident in physical memory and cannot be swapped to the disk) and there is no memory protection (noSIGSEGV signals, unlike in user space), therefore memory violations lead to instability and system crashes.[14]: 20 User memory is pageable by default, although paging for specific memory areas can be disabled with themlock()system call family.
Page frame information is maintained in apposite data structures (of typestruct page) that are populated immediately after boot and kept until shutdown, regardless of whether they are associated with virtual pages. The physical address space is divided into different zones, according to architectural constraints and intended use. NUMA systems with multiple memory banks are also supported.[286]
Small chunks of memory can be dynamically allocated in kernel space via the family ofkmalloc() APIs and freed with the appropriate variant ofkfree().vmalloc() andkvfree() are used for large virtually contiguous chunks.alloc_pages() allocates the desired number of entire pages.
The kernel used to include the SLAB, SLUB and SLOB allocators as configurable alternatives.[288][289] The SLOB allocator was removed in Linux 6.4[290] and the SLAB allocator was removed in Linux 6.8.[291] The sole remaining allocator is SLUB, which aims for simplicity and efficiency,[289] isPREEMPT_RT compatible[292] and was introduced in Linux 2.6.
Since Linux supports numerous filesystems with different features and functionality, it is necessary to implement a generic filesystem that is independent from underlying filesystems. Thevirtual file system interfaces with other Linux subsystems, userspace, or APIs and abstracts away the different implementations of underlying filesystems. VFS implements system calls likecreate,open,read,write andclose.VFS implements a genericsuperblock[293] andinode block that is independent from the one that the underlying filesystem has.
In this subsystem directories and files are represented by astruct filedata structure. Whenuserspace requests access to a file it is returned afile descriptor (non negative integer value) but inkernel space it is astruct file structure. This structure stores all the information the kernel knows about a file or directory.
sysfs andprocfs are virtual filesystems that expose hardware information anduserspace programs' runtime information. These filesystems aren't present on disk and instead the kernel implements them as acallback or routine which gets called when they are accessed by userspace.
While not originally designed to beportable,[17][294] Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from theARM architecture to IBMz/Architecturemainframe computers. The first port was performed on theMotorola 68000 platform. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as afork and a "Linux-like operating system".[294] However, that moved Torvalds to lead a major restructure of the code to facilitate porting to more computing architectures. The first Linux that, in a single source tree, had code for more than i386 alone, supported theDECAlpha AXP 64-bit platform.[295][296][294]
Linux runs as the main operating system onIBM'sSummit; as of October 2019[update], all of the world's500 fastest supercomputers run some operating system based on the Linux kernel,[71] a big change from 1998 when the first Linux supercomputer got added to the list.[297]
Linux has also been ported to various handheld devices such asApple'siPhone 3G andiPod.[298]
In 2007, the LKDDb project has been started to build a comprehensive database of hardware and protocols known by Linux kernels.[299] The database is built automatically by static analysis of the kernel sources. Later in 2014, the Linux Hardware project was launched to automatically collect a database of all tested hardware configurations with the help of users of various Linux distributions.[300]
This section needs to beupdated. Please help update this article to reflect recent events or newly available information. Last update: April 2015(September 2023)
Rebootless updates can even be applied to the kernel by usinglive patching technologies such asKsplice,kpatch andkGraft. Minimalistic foundations for live kernel patching were merged into the Linux kernel mainline in kernel version 4.0, which was released on 12 April 2015. Those foundations, known aslivepatch and based primarily on the kernel'sftrace functionality, form a common core capable of supporting hot patching by both kGraft and kpatch, by providing anapplication programming interface (API) for kernel modules that contain hot patches and anapplication binary interface (ABI) for the userspace management utilities. However, the common core included into Linux kernel 4.0 supports only thex86 architecture and does not provide any mechanisms for ensuringfunction-level consistency while the hot patches are applied.
Kernel bugs present potential security issues. For example, they may allow forprivilege escalation or createdenial-of-service attack vectors. Over the years, numerous bugs affecting system security were found and fixed.[301] New features are frequently implemented to improve the kernel's security.[302][303]
Capabilities(7) have already been introduced in the section about the processes and threads. Android makes use of them andsystemd gives administrators detailed control over the capabilities of processes.[304]
Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security which are collectively known as theLinux Security Modules (LSM).[305] They comprise theSecurity-Enhanced Linux (SELinux) module, whose code has been originally developed and then released to the public by theNSA,[306] andAppArmor[193] among others. SELinux is now actively developed and maintained onGitHub.[192] SELinux and AppArmor provide support to access control security policies, includingmandatory access control (MAC), though they profoundly differ in complexity and scope.
Another security feature is the Seccomp BPF (SECure COMPuting with Berkeley Packet Filters) which works by filtering parameters and reducing the set of system calls available to user-land applications.[307]
Critics have accused kernel developers of covering up security flaws, or at least not announcing them; in 2008, Torvalds responded to this with the following:[308][309]
I personally consider security bugs to be just "normal bugs". I don't cover them up, but I also don't have any reason what-so-ever to think it's a good idea to track them and announce them as something special...one reason I refuse to bother with the whole security circus is that I think it glorifies—and thus encourages—the wrong behavior. It makes "heroes" out of security people, as if the people who don't just fix normal bugs aren't as important. In fact, all the boring normal bugs areway more important, just because there's[sic] a lot more of them. I don't think some spectacular security hole should be glorified or cared about as being any more "special" than a random spectacular crash due to bad locking.
Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offerlong-term support releases that receive security updates for a certain Linux kernel version for an extended period of time.
In 2024, researchers disclosed that the Linux kernel contained a serious vulnerability, CVE-2024-50264, located in the AF_VSOCK subsystem. This bug is a use-after-free flaw, a class of memory corruption issue that occurs when a program continues to use memory after it has been freed.[310][311] Such flaws are particularly dangerous in the kernel, as they can allow attackers to escalate privileges. The bug was resolved in May 2025.[312]
Initially, Torvalds released Linux under a license which forbade any commercial use.[313] This was changed in version 0.12 by a switch to theGNU General Public License version 2 (GPLv2).[22] This license allows distribution and sale of possibly modified and unmodified versions of Linux but requires that all those copies be released under the same license and be accompanied by - or that, on request, free access is given to - the complete corresponding source code.[314] Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".[313]
The Linux kernel is licensed explicitly underGNU General Public License version 2 only (GPL-2.0-only) with an explicit syscall exception (Linux-syscall-note),[8][11][12] without offering the licensee the option to choose any later version, which is a common GPL extension. Contributed code must be available underGPL-compatible license.[13][204]
There was considerable debate about how easily the license could be changed to use later GPL versions (including version 3), and whether this change is even desirable.[315] Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2.[316] However, the terms of the GPL state that if no version is specified, then any version may be used,[317] andAlan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL.[318]
In September 2006, a survey of 29 key kernel programmers indicated that 28 preferred GPLv2 to the then-current GPLv3 draft. Torvalds commented, "I think a number of outsiders... believed that I personally was just the odd man out because I've been so publicly not a huge fan of the GPLv3."[319] This group of high-profile kernel developers, including Torvalds,Greg Kroah-Hartman andAndrew Morton, commented on mass media about their objections to the GPLv3.[320] They referred to clauses regardingDRM/tivoization, patents, "additional restrictions" and warned aBalkanisation of the "Open Source Universe" by the GPLv3.[320][321] Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.[322]
It is debated whether someloadable kernel modules (LKMs) are to be consideredderivative works under copyright law, and thereby whether or not they fall under the terms of the GPL.
In accordance with the license rules, LKMs using only a public subset of the kernel interfaces[229][230] are non-derived works, thus Linux gives system administrators the mechanisms to load out-of-tree binary objects into the kernel address space.[13]
There are some out-of-tree loadable modules that make legitimate use of thedma_buf kernel feature.[323] GPL compliant code can certainly use it. However, a different possible use case would beNvidia Optimus that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into theIntel framebuffer when it is active. But, Nvidia cannot use this infrastructure because it necessitates bypassing a rule that can only be used by LKMs that are also GPL.[231]Alan Cox replied onLKML, rejecting a request from one of Nvidia's engineers to remove this technical enforcement from the API.[324] Torvalds clearly stated on the LKML that "[I] claim that binary-only kernel modules ARE derivative "by default"'".[325]
On the other hand, Torvalds has also said that "[one] gray area in particular is something like a driver that was originally written for another operating system (i.e., clearly not a derived work of Linux in origin). THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour".[326]Proprietary graphics drivers, in particular, are heavily discussed.
Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted",[327] and therefore bug reports from tainted kernels will often be ignored by developers.
The official kernel, that is, Torvalds's git branch at the kernel.org repository, containsbinary blobs released under the terms of the GNU GPLv2 license.[8][13] Linux can also load binary blobs, proprietary firmware, drivers, or other executable modules from the filesystem, and link them into kernel space.[328]
When necessary (e.g., for accessing boot devices or for speed), firmware can be built-in to the kernel, meaning building the firmware intovmlinux; however, this is not always a viable option for technical or legal issues (e.g., it is not permitted to do this with firmware that is not GPL compatible, although this is quite common nonetheless).[329]
Linux is a registeredtrademark of Linus Torvalds in the United States, the European Union, and some other countries.[330][331] A legal battle over the trademark began in 1996, when William Della Croce, a lawyer who was never involved in the development of Linux, started requesting licensing fees for the use of the wordLinux. After it was proven that the word was in common use long before Della Croce's claimed first use, the trademark was awarded to Torvalds.[332][333][334]
In October 2024, during theRussian invasion of Ukraine, kernel developerGreg Kroah-Hartman removed some kernel developers whose email addresses suggested a connection to Russia from their roles as maintainers.[335][336]Linus Torvalds responded that he did not support Russian aggression and would not revert the patch, insinuating that opponents of the patch wereRussian trolls.[337] James Bottomley, a kernel developer, issued an apology for the handling of the situation and clarified that the action was a consequence ofU.S. sanctions against Russia.[338]
^As a whole, Linux source is provided under the terms of theGPL-2.0-only license with an explicit syscall exception.[11][12] Aside from that, individual files can be provided under a different license which is required to be compatible with the GPL-2.0-only license (i.e., the GNU General Public License version 2) or a dual license, with one of the choices being the GPL version 2 or a GPLv2 compatible license.[13]
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